A gyro gunsight is a modification of the non-magnifying reflector sight

Reflector sight

A reflector or reflex sight is a generally non-magnifying optical device that allows the user to look through a partially reflecting glass element and see an illuminated projection of an aiming point or some other image superimposed on the field of view...

 in which target lead (the amount of aim-off in front of a moving target) and bullet drop are allowed for automatically, the sight incorporating a gyroscopic mechanism that computes the necessary deflection

Deflection

Deflection or deflexion may refer to:* Deflection , the displacement of a structural element under load* Deflection , a technique of shooting ahead of a moving target so that the target and projectile will collide...

s required to ensure a hit on the target. The sight was developed just before the Second World War for aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 use during aerial combat.

The sight features both a fixed and a moving graticule (aiming point)

• The fixed one signifying the direction the guns are pointing (in effect, the same as a "normal", non-gyro, sight).
• The moving one the corrected aiming point.

Providing the pilot/gunner uses the correct moving graticule then a hit on the target is highly likely.
Gyro gunsights were modifications of the reflector gunsight to aid pilots in hitting targets (other aircraft) that were turning rapidly in front of them.